Millions of people in the U.S. with diabetes depend on a reliable supply of insulin. Although insulin can be readily purchased, and has a reasonable shelf life, it needs to be stored under proper conditions. Insulin is a protein that can be degraded by exposure to excessive heat or cold. In particular, even brief exposure to temperatures below 2° C. or above 30° C. can cause unacceptable degradation. In general, it is recommended that insulin be stored in a refrigerator for extended storage, but room temperature storage for periods up to a month is also considered acceptable, provided that the room temperature does not exceed 30° C. (86° F.). Many insulin users are satisfied with keeping a supply in their refrigerator. However, refrigerators occasionally are set at a temperature too low for safe storage of insulin, and insulin can accidentally be frozen. In addition, insulin users who travel regularly are often faced with issues, such as hotel rooms that do not provide refrigerators, and long airplane flights. During travel in such unpredictable environments, it is possible that the insulin could be exposed to damaging temperatures, even without the knowledge of the owner.
The only devices currently on the market aimed at providing portable personal refrigerated insulin storage are phase-change devices. These are devices that contain a fluid/solid that melts near 10° C.; they are essentially ice packs. The fluid might be water, or some water-based fluid. Like an ice pack, prior to use, they must be pre-chilled in a freezer. The heat of fusion absorbed as the solid melts is used to keep it cold for an extended period.
Several difficulties are encountered with these devices. Among these are that temperature control is marginal; the device starts at freezer temperature (which is too low for safe insulin storage) and gradually warms to the phase change temperature. The temperature then remains fairly constant until all of the phase-change material melts, at which time the temperature begins to rise again. If the phase-change material is water, the temperature plateau is at 0° C., which is again too cold for long-term insulin storage. Phase-change materials can be found that melt at 10° C., but they do not have the high heat of fusion of water, limiting the lifetime of the device. Lifetime of the devices is also limited if the mass of the phase-change material is to be kept reasonable. In addition, there is no reliable indicator of when the solid is nearly or completely exhausted. The user only knows it is time to recharge the device when it begins to get too warm. Finally, the only way to recharge the device is to leave it in a freezer for some time; the user is then faced with the issues of finding a freezer, and of where to keep the insulin while the storage device is being recharged (since it is not safe to leave the insulin in the device while it is in the freezer).
It would be useful to be able to provide a portable refrigeration device that helps prevent unacceptable degradation of a substance stored therein.